In recent years, automotive exhaust emission control system performance has become an important issue across the U.S. Virtually all cars sold in the U.S. from the early 1980's have been equipped with a three-way catalytic converter in the exhaust system. In order for this catalytic converter to function correctly, the vehicle is also typically equipped with a fuel control system which maintains a stoichiometric mixture (i.e. Air mass/fuel mass= 14.7).
The long-term performance of automotive exhaust emission control systems is strongly influenced by the physical condition of the catalytic converter. Unfortunately, the catalytic converter is susceptible to irreversible damage from any number of factors.
One of the most likely causes of catalyst degradation is the occurrence of engine misfire. Misfire is a condition in which combustion does not occur in one or more engine cycles in one or more cylinders due, for example, to absence of ignition, or misfueling. Under engine misfire conditions, unburned fuel and air are pumped into the catalyst, greatly increasing its operating temperature. This problem is usually most severe under high load, high speed engine operating conditions, where even a few seconds of misfire can cause catalyst temperatures to soar above 900.degree. C. (1650.degree. F.), causing irreversible damage to the catalyst. Even today's most advanced catalysts generally are unable to sustain continuous operation above 900.degree. C. without damage.
Vehicle operation while misfire is present also contributes to excess emissions, especially when the misfire is present during engine warmup and the catalyst has not reached operating temperature. Obviously, misfire is also undesirable because the engine produces reduced or no torque during the misfiring cycle.
The integrity of the exhaust emission system can best be maintained by monitoring its performance continuously on board the vehicle. It is with the intent of monitoring emission system performance that the California Air Resources Board in 1989 passed regulations which will require all new vehicles after 1994 to be equipped with on-board monitoring systems capable of detecting misfires. These proposed regulations are known as OBDII and may be followed by similar Federal EPA regulations. The proposed regulations are applicable for any misfire condition (e.g. random, continuous, equally spaced, etc.) for the purpose of identifying a malfunction.
There are a variety of methods and systems for detecting misfire. These include the use of crankshaft angular velocity fluctuation, observing the change in oxygen sensor waveform pattern, enhancing the present knock sensor concept to "listen" for the absence of combustion, installation of cylinder pressure transducers, analysis of secondary ignition waveform pattern, use of temperature sensors to detect catalyst temperature during misfire, and others.
The prior art discloses many methods of detecting misfire based upon measurements of torque as derived from noncontacting crankshaft angular velocity measurements. The misfire condition is detected from these torque measurements. These methods of torque measurement are well known. However, each of these methods has certain deficiencies with respect to cost effective, reliable misfire detection as required by the OBDII regulations.
For example, each of U.S. Pat. Nos. 4,843,870, 4,697,561 and 4,532,592 disclose a method of measuring engine torque utilizing digital techniques to sample crankshaft angular velocity. The time between successive fixed angular positions on the crankshaft is measured using a high frequency clock. One sample of crankshaft angular velocity .omega..sub.i obtained by the relation ##EQU1## where .theta..sub.i and .theta..sub.i-1 are the crankshaft angular positions. .DELTA.t.sub.i =time interval.
One of the problems of this digital measurement of angular velocity is the random (or pseudorandom) errors involved. There are two error sources for the digital method: (1) the random variations in measurement of angular position and (2) the timing errors involved in measuring .DELTA.t.sub.i. The first error source results from runout of the crankshaft gear and by variations in the magnetic coupling of the sensor to the crankshaft gear. In order to be practical for torque measurements, these angular velocity measurements must be filtered by means of a digital filter.
Thus the digital measurement of crankshaft angular velocity has the disadvantage of requiring electronic complexity simply to obtain a measurement (with minimum random error) of crankshaft angular velocity. In addition, this digital method has a limited sampling rate which is influenced by the angular separation .DELTA..theta. (.DELTA..nu.=.theta..sub.i -.theta..sub.i-1). The clock frequency must be extremely high to have an adequate number of counts to achieve the desired sampling rate and achieve the accuracy required to measure .DELTA..theta.. The accuracy of determining .theta..sub.i and .DELTA.t.sub.i decreases with increasing RPM.
Another deficiency in the digital method is the rather cumbersome method of dealing with engine dynamics. It has long been recognized that calculation of torque from crankshaft angular velocity measurements requires a correction for the forces associated with reciprocating components (i.e. piston, connecting rod). This is illustrated in the reference A. Rizzoni, "A Model for the Dynamics of the Internal Combustion Engine", PhD dissertation, Department of Electrical and Computer Engineering, University of Michigan, Ann Arbor, Mich., February 1986.
Still another deficiency in the references pertaining to torque nonuniformity measurements relative to misfire detection is the lack of any recognition that an index of torque nonuniformity is a random variable. There is no mention of the random nature of engine torque in the prior art references except for those published by the present inventors.
For example, U.S. Pat. No. 4,550,595 discloses an analog circuit-based method of continuously estimating the instantaneous indicated torque of a four cylinder, two stroke/cycle reciprocating internal combustion engine. This patent teaches a method of calculating this torque based upon noncontacting continuous time measurements of crankshaft angular velocity. An exact calculation which accounts for the influence of the inertial forces associated with the reciprocating components on the crankshaft angular dynamics is also taught. There is no suggestion of using the measured torque for any cylinder by cylinder performance measurement and there is no hint of misfire monitoring.
U.S. Pat. No. 3,789,816 discloses a closed loop fuel control system for gasoline fueled reciprocating internal combustion engines. The control system incorporates instrumentation for measuring "engine roughness" (i.e. cylinder to cylinder and cycle to cycle torque imbalance). The roughness signal is obtained by electronic signal processing of crankshaft angular velocity measurements. The electronic signal processing does not account for reciprocating inertia forces.
U.S. Pat. No. 4,292,670 discloses a method for measuring the power and/or compression balance for a diesel engine. The method uses a noncontacting sensor to obtain a signal from the starter ring gear. Using a digital method, estimates are obtained of crankshaft angular velocity with a very limited sampling rate. The angular velocity measurements are then used as a means of estimating the work done by the engine during power stroke without compensating for reciprocating inertial forces in the engine dynamics.
U.S. Pat. No. 4,197,767 discloses a method of fuel control for a gasoline fueled IC. engine during warm-up period. A method of measuring "engine roughness" is provided. This method incorporates a noncontacting sensor for measuring crankshaft angular velocity. Electronic signal processing generates a signal which is indicative of engine roughness. However, there is no teaching of reciprocating inertia compensation. Furthermore, there is no hint of any relationship between the engine roughness signal and actual engine misfire.